All organism need energy to perform various processes of their lives. Energy is necessary for them whether they are working or at rest. We require energy even during sleep because a number of biological activities occur continuously in our body. For example, respiration and beating of heart keeps on going always without any break. All these processes of living beings need a continuous flow of energy. Food is the source of this energy. It is released by the breakdown of food taken by the organism. In the forthcoming text, we study various modes of intake of food and nutrition.

Nutrition:The very word nutrition means the process of taking in and utilizing the various nutrients by an organism. Nutrients are substance that provide energy to the organism. They also help in the biosynthesis of various consituents of the body. These are obtained by organisms from their surroundings. Food an important source of various nutrients required for growth, maintenance and repair of the body. It is also required for building the resistance in the body again various diseases and foregin pathogens. Thus, nutrition is a process by which organism obtains substances which are utilized for their growth, maintenance, reproduction and to counteract various diseases. Substances used for nutrition are called nutrients.